Learning to plot a story comes next. I wish I could say plotting is easy for me, but it really isn't. Let's talk about two basic ways at arriving at a plot.
The first way, which a lot of excellent writers use, such as Ursula LeGuin and John C. Wright, is to plot toward the horizon, as if one were walking. You walk (or follow your characters) as far as you can see (i.e. plot), until you reach the horizon. Once there, with part of the plot behind you, you can see a little farther and know what might happen next. This is the way I have plotted all my previous novels. The advantage to it is that you don't spend as much time plotting before you ever start writing.
The second way is to sit down and plot a novel from start to finish. I did this on my current novel and will probably never go back to doing it the other way. The process is fairly simple, but I believe it has to begin with characterization. Until you know who your characters are, how can you know how they will react in a given situation? If you do know your characters before you begin writing, you can place them in a situation and see what they do. It then becomes a matter of 'what iffing' your way to the end of the story. "What if my character falls down a rabbit hole leading to a fantasy world?" "What if she is taken prisoner by Nazis?" The 'what ifs' are a logical sequence. "If this happens, what are the consequences?" To create a plot you work through the sequence, perhaps creating many branching sequences to see which one feels "best."
This isn't quite as easy as it sounds, however. The way I did it on my last book, which is now in its second draft, was to use a pencil and notebook (primitive, I know). I knew more or less what would happen in my first chapter, because I had already been thinking about it awhile. So after creating my main character, I started jotting ideas, what iffing along, putting down pretty much anything I thought of, forcing myself to work a couple of hours a day. Sometimes I went longer, sometimes shorter. There were days when I was completely dry--nothing would come, and others where I had all sorts of ideas. Regardless, I had to force myself to keep at it, waiting for "breakthroughs." At times it was discouraging; I had to reassure myself to keep believing in the process.
At first the plotting was very linear--I was at Chapter One and had nowhere to go but forward; but as I got farther into the story, I would bounce around to different parts of the book, adding details here and there. It's an odd process.
It's important at this stage not to try to edit your thoughts; you will eventually discard many of your ideas, but you don't want to limit yourself at this point. In fact, what happened for me was a gradual narrowing. One day I would think: "Maybe my characters can do this. If they did, then this might happen, which would cause this to happen." A day or two later, that path would have settled in my mind, and I would be thinking: "My characters will do this." The transition from maybe to definitely occurred almost without my realizing it. You continue along until the book begins to take shape. You want to be writing all your ideas down, even if you eventually discard many of them. It makes the process work, and rereading the previous day's thoughts will often lead you forward.
When plotting, how do you come up with events that are exciting? Let's look at a simple illustration I stole from somewhere: You're writing about a cat that is caught in a tree. Not very interesting. Let's up the suspense. The tree is on fire. The cat's owner is below, trying to save the cat. Still not terribly interesting—he could probably call the fire department. Up the suspense again—before calling the fire department, he tries to climb the tree, falls, and breaks his leg. The cat has only a few moments to live; the owner has a broken leg and no cell phone. There is no one around to help him.
At this point, unless you're more imaginative than I, you don't know how he is going to save the cat. One of the guidelines here is that he has to do so without outside help (unless he finds an ingenious way to call for it). The hero has to be the one to triumph (or gloriously fail).
This is the part that separates professional writers from amateurs. The only way to solve the problem is to present it to your subconscious until it finds a solution, just as I was doing when I plotted my novel. (Even when I wasn't consciously working on the plot of my novel, it remained in the back of my mind.) You let the dilemma sit, like an aching tooth, in the back of your brain. At some moment when you're shaving or driving or walking, the answer will come to you. This is harder work than one might think, because at some level your mind is engaged on this almost constantly. If it isn't, you have to keep reminding it to do the work. (You will, by the way, also find your spouse asking questions like: "Are you plotting?" when you haven't spoken for thirty minutes.
The key to this is: if you struggle to come up with a solution instead of picking an easy one, your reader will be surprised at the way the owner saves the cat. This will make you look really clever.
I notice this process most when writing short stories—I throw out a premise, write my character into lots of trouble, and then reach a point where I don't know what to do next. Sweat beads my brow—I stand on a dizzying height without a net-because there is always the chance I may not be able to find an ending to the story. Sometimes I don't, at least one I feel is satisfying. Most of the time I do. And it's the difference between an interesting story and one where the reader begins to skim. I think one of the problems about genres such as Sword and Sorcery is a lack of surprise. The barbarian swordsman is faced with a monster/dragon/enemy. After much sound and fury, he defeats his foe. Some of this is fun, but if the writer can add another dimension beyond the physical—for example, if defeating his foe will cause the death of his best friend. Now, there's a problem to be solved—maybe he lets his best friend die; maybe he finds a way to prevent it. The story is deepened.
Which brings up another point about writing—someone once said that "all stories are moral." Whether that's true or not, if there is a moral issue in a story, it makes it stronger. Betraying a friend, performing an act of murder, choosing between two evils. I was watching an episode of the old TV show Bonanza, the first one I had seen since I was a kid, and I was taken by the simple morality used to draw me in. Bonanza, of course, is a cowboy show, and in this episode Hoss Cartwright agrees to go to a neighboring town and escort a young woman back to Virginia City to be the bride of a forty-something year old rancher. The rancher and the woman have never met; their only correspondence has been through letters. In fact, this poor girl, living in the back woods, hasn't met any men at all. Hoss, the innocent, shy cowboy, going with the best of intentions, is oblivious when the woman develops a crush on him. Returning to Virginia City, she refuses to marry the rancher, believing Hoss must be in love with her. The rancher is furious, our hero is in trouble; mayhem ensues.
You read the above and chuckle. How simple and naive TV was in those days! I couldn't stop watching until I found out how it ended. Because Hoss was the innocent, blamed for something he hadn't done, and I had to know how he would work through the problem. Because we've all, at some point, been accused of something we haven't done.
Granted, this is not deep writing—the problem is that it is based on a misunderstanding, which a bit of communication could have cleared up at once. They loved misunderstandings in the early days of TV—just watch any episode of I Love Lucy. But if you think moral dilemmas can't work in writing today, I have only to mention Orson Scott Card. Read the moral dilemmas throughout his novels, such as Ender's Game and its sequels, and see if you're not riveted.